This invention relates to gas turbine engines and, more particularly, to acoustically-treated inlet structure for supplying inlet air to a gas turbine engine while sumultaneously suppressing noise travelling outwardly through the air inlet.
It is known within the acoustic art that sound energy (e.g., noise) travelling through a duct or airflow channel can be attenuated or suppressed by acoustically absorptive lining materials mounted along the interior wall surfaces of the duct. Further, it is known that, when such acoustic treatment is applied to the duct walls, the acoustically treated area necessary to provide a given amount of sound suppression is directly proportional to the dimensions of the duct, e.g., duct height or diameter.
These acoustic principles have been applied in part to the gas turbine engine art in an attempt to suppress the noise generated by a gas turbine engine. For example, one component of gas turbine noise that is commonly called inlet noise is due to acoustic energy propagating in the forward direction through the air inlet duct of a gas turbine engine. Inlet noise is due to a number of sources including the engine compressor stage and the high speed rotation of the fan in a turbofan engine including fan blade tip vortices. Since forwardly propagating inlet noise constitutes a considerable portion of the overall noise level generated by a gas turbine engine, especially during the approach and landing maneuvers of jet aircraft employing such gas turbine engines, considerable effort has been devoted to the suppression of engine noise in the inlet structure. Since completely lining the air inlet wall area of a conventional gas turbine engine with sound suppressing liners does not generally provide adequate noise suppression, much of the prior art is directed to structural methods of increasing the available mounting area and/or decreasing the effective airflow channel dimensions within the inlet duct to thereby decrease the required amount of acoustically absorptive lining material.
One common approach to increasing the area available for mounting acoustically absorptive liners while simultaneously reducing the dimensions of at least a portion of the inlet duct is the use of one or more thin rings coaxially mounted within the air inlet. In most configurations in which more than one such ring is employed, the rings are concentrically mounted about the axial center line of the air inlet with acoustically absorptive material lining the surfaces of the rings. See, for example, "Progress of NASA Research Relating to Noise Alleviation of Large Subsonic Jet Aircraft," NASA Report SP-189, 1968, Paper #9, "Design Concepts," by Robert E. Pendley.
Although such rings effectively subdivide a portion of the air inlet into smaller airflow channels that are more amenable to low noise operation and provide a somewhat increased area for the mounting of acoustically absorptive lining material, such techniques, have not reduced the inlet noise to a suitable level. Thus, as stricter governmental regulations on engine noise level have been imposed, it has become the practice in the art to provide longer inlet ducts to further increase the area available for the mounting of acoustically absorptive linings. In this respect, modern acoustically treated inlet ducts often have a length-to-diameter ratio on the order of 1 or more, whereas the air inlets of gas turbine engines employed prior to the present-day noise restrictions commonly had length-to-diameter ratios of approximately 0.6.
Increasing the length of the air inlet structure generally causes a detrimental increase in the weight of the air inlet. Further, relatively long inlet ducts are undesirable from an aerodynamic standpoint, since such relatively long air inlets are subjected to substantial airloads as the aircraft is operated. For example, since the air inlet is generally mounted to project forwardly from the front face of the engine, climbing maneuvers, such as experienced during aircraft take-off procedures, couple a substantial bending moment to the engine and to the structure upon which the engine is mounted. Thus, such relatively long, acoustically-treated inlets place further constraints on the structural design of the engine in that the engine and its mounting arrangement must be able to withstand the additional loading. Designing an air inlet to withstand this structural loading often results in a further increase in inlet weight. Additionally, it will be recognized that such relatively long air inlet structure may not be suitable for use in aircraft designs wherein the engine is mounted in close proximity with other aircraft structure. Accordingly, it can be said that, although the relatively long prior art acoustically inlets have provided improvement in noise performance, this improvement has introduced performance penalties and design constraints both in the design of gas turbine engines and in the design of aircraft employing such engines.
The structure of the rings used within such prior art inlets also present a number of disadvantages and drawbacks. First, to minimize the blockage of air flowing through the air inlet and to minimize the turbulence created by the rings, the rings generally have a rather thin cross-sectional geometry. This geometry creates problems in that such thin rings must be constructed of relatively heavy material such as steel in order to attain sufficient structural integrity. Further, it is difficult to prevent the formation of ice on such thin rings with conventional aircraft thermal anti-icing systems. Additionally, even when constructed of steel or other strong material, such rings are extremely vulnerable to the impact of foreign objects, such as birds, and often break under impact. When such breakage occurs, the ring fragments usually pass into the engine turbo-machinery to severely damage or destroy the engine. In addition to the above-noted drawbacks, the mounting of such rings within the air inlet tends to prevent visual inspection of the forward mounted engine components, such as the fan blades of a turbofan gas turbine engine.
There is yet another problem that relates to air inlets of both the acoustically-treated variety and those air inlets which are not equipped for noise suppression which occurs when strong cross-winds or wind shear are encountered. Under such conditions, air does not pass directly into and through the airflow channel of conventional air inlets and localized pressure disturbances are created throughout the aircraft engine. Such pressure disturbances detrimentally affect engine performance and can, under severe cross-wind conditions, cause at least momentary engine failure.
Accordingly, it is an object of this invention to provide a gas turbine engine air inlet having a relatively low length to diameter ratio which is configured for the suppression of inlet noise.
It is another object of this invention to provide a low noise, gas turbine engine air inlet wherein the weight and performance penalties normally attendant with such a low noise duct are minimized.
It is still another object of this invention to provide a gas turbine air inlet that is configured to increase the area available for the mounting of acoustically absorptive material while simultaneously decreasing the dimensions of the airflow channels within the air inlet to thereby provide an engine inlet wherein inlet noise can be efficiently suppressed by means of acoustic lining materials.
It is yet another object of this invention to provide a low noise, gas turbine air inlet compatible with conventional aircraft anti-icing systems.
Even further, it is an object of this invention to provide a noise suppressing air inlet having less obstruction to the visual inspection of the front face of a gas turbine engine utilized therewith.
A further object of the invention is to provide an acoustically-treated air inlet which inproves aircraft engine performance under cross-wind conditions.